1. Field of the Invention
The present invention relates to an optical fiber fixing method for fixing a ferrule, which is a terminal component for optical communication, and an optical fiber to each other.
2. Description of the Related Art
As well known, a method of connecting optical fibers to each other is roughly classified into a permanent connection in which they cannot be disconnected after once connected, and a connector splicing in which they can be freely disconnected even after being once connected. The permanent connection is utilizes a technique of fusion splicing or bonding splicing. The connector splicing fits connectors (one is a plug, and the other is a receptacle) to each other so that the end surfaces of optical fibers are physically spliced to each other.
The latter connector connection has a disadvantage of great connection loss of the optical fiber resulting from axis displacement, axis inclination, etc., but has an advantage of a short splicing time. Therefore, the connector splicing has been widely employed as a connecting technique for short-distance optical communication, e.g., a technique of communication devices within a vehicle such as a motor car.
The optical connector for connector splicing includes an optical fiber serving as a communication line and a ferrule serving as a terminal component. The ferrule is employed to position the optical fiber in an axial direction and a radial direction, and to fix the terminal side of the optical fiber. The technique for splicing the optical fiber and ferrule is implemented in various manners, in particular, in a manner of using adhesive.
An explanation will be given of the method of splicing the optical fiber and ferrule using adhesive. The optical fiber includes a fiber wire composed of a core and cladding, and a coating. On the tip side of the optical fiber, the coating is peeled to expose the fiber wire.
The ferrule is formed in a cylindrical shape. The through-hole formed internally has a small diameter portion and a large diameter portion. The small diameter portion has a diameter large enough to insert the fiber wire. The large diameter portion has a diameter large enough to insert the coating. The tip of the fiber wire passed into the small diameter portion is ground to be flush with the end surface of the ferrule.
The ferrule and optical fiber can be made of various materials, e.g., quartz glass or synthetic resin. Further, the ferrule may be made of metal or ceramic.
The optical fiber and ferrule are fixed to each other in such a manner that adhesive is applied to the coating of the optical fiber, the optical fiber is inserted into the ferrule and the adhesive is hardened.
However, the fixing method using the adhesive takes a long time to heat and harden the adhesive. This presents problems of low productivity of the optical connector, of changes in the bonding strength due to the surface property (surface wettability, surface coarseness, etc.) and of low heat resistance. JP-A-1′-142688 has proposed a technique for solving these problems.
As seen from FIG. 5, by irradiating the end surface 60a of an optical fiber 60, inserted in a through-hole 61a of a ferrule 61, with a laser beam 63, the boundary 62 between the ferrule 61 and optical fiber 60 is molten with thermal energy of the laser beam 63 so that the ferrule 61 and the optical fiber 60 are fixed to each other.
However, the conventional fixing method presents the following problem to be solved.
Firstly, because the ferrule 63 is irradiated with the laser beam 61 from its end surface, the molten area is so small that fixing cannot be assured. Therefore, if tensile stress acts on the optical fiber 60, the optical fiber 60 may detach from the ferrule 61.
Secondly, even when the laser beam 63 is applied from the side of the ferrule, according to the manner of applying the laser beam 63, the laser beam 63 is not uniformly applied so that the fixing force is not constant. Specifically, in some places, the laser beam 63 is strongly applied and the fiber wire may be damaged, whereas in other places, the laser beam 63 is weakly applied and the fiber wire may be insufficiently heated. The reason why the laser beam 63 is applied strongly or weakly is that the applying distance of the laser beam 63 varies according to places.
If the applied area becomes large because the laser beam is out of focus, the layer susceptive to thermal influence extends outwardly. In this case, the fiber wire consisting of a core and cladding may generate thermal cracks. The layer susceptive to thermal influence can be narrowed by decreasing the output of the laser beam. However, in this case, the heating becomes insufficient so fixing cannot be assured.